The projects that will be undertaken are: 1) Activation of delayed implanting mouse blastocysts in vitro appears to involve a significant increase in the rate of embryonic RNA synthesis. It is not clear whether serum factors in the uterine lumen are important in stimulating the increase and this will be tested by measuring the incorporation of 3H uridine (as well as the specific activity and size of the embryonic UTP pool) by delayed implanting mouse blastocysts in the presence and absence of serum. 2) Mintz has proposed that uterine protease may act as an implantation initiation factor by lysing the zona pellucida and changing blastocyst surface properties causing them to adhere to the uterine wall. We have demonstrated the existence and described the endocrine control of such an enzyme and will now attempt to determine some of its physical and chemical characteristics. 3) DNA polymerase increases with the onset of rapid cell division in preimplantation mouse embryos. The amount of enzymes in normal and delayed implanting mice will be compared to determine whether or not the level of activity could be a regulatory factor in the dormant condition of delayed implanting embryos.